Cyber Monday got off to a solid start with online sales reaching $531 million as of 10 a.m. Eastern. Based on the initial numbers from data generated by about 80 of the top U.S. online retailers, Adobe Analytics predicts a $7.8 billion day at 18.3% growth year over year (YoY).

The prediction makes the online shopping day the biggest in U.S. history, with smartphones contributing more than $2 billion in online sales for the day.

But retailers could have seen higher numbers during the weekend. Adobe estimates out-of-stock products cost retailers an estimated $140 million in lost online sale for Nov. 24 and Nov. 25. That estimate puts losses at $120 million on Thanksgiving and about $177 million on Black Friday. 

More than 2% of product page visits saw out-of-stock messages during the holiday weekend, up slightly over the season average of 2.12%, but below Thanksgiving at 3.26% and Black Friday at 2.85%.

Come Monday morning, Cyber Monday shoppers relied heavily on their mobile devices. Smartphones accounted for 58.9% of visits to retail sites, up 32.1% YoY. Desktop contributed 34.7%, and tablets 6.4%.

While online shopping shows sizable gains this year, the hours between 10:00 p.m. and 1:00 a.m. Eastern should bring in $1.6 billion, about $200 million more compared to an average full day in the year, according to Adobe.

Conversion rates are expected to peak above 7.3% for the season, about 2.5 times higher compared with the rest of the year.

Adobe forecasts that consumers have spent $50.6 billion online between November 1 and November 25, 2018, accounting for 20% growth YoY.

Perhaps a revised upward advertising outlook can explain some of the increase in online purchases. Pivotal analyst Brian Wieser on Monday revised his 2018 U.S. ad-growth estimates 1.7 percentage points to 6.5%, and projects that 2019 will expand 4% versus his previous forecast of 3.5% for 2019. The lower estimate was a result of slower growth in the third quarter of 2018, he wrote.

Wieser cited several economic indicators for the upward revision such as the higher personal consumption expenditure growth rate, and an increase of industrial production.

Consumers buying through smartphones contributed about $15.3 billion, up 56% YoY. 

Thanksgiving drove $3.7 billion, up 28% YoY; and Black Friday about $6.22 billion, up 23.6% YoY.

Interestingly, Adobe reported that Denver had the biggest shopping baskets since Thanksgiving with orders averaging $163, compared with the U.S. average of $138. San Francisco came in at $157 and New York at $157. 

Shopify’s data for holiday sales tracked high with estimates that weekend sales rose about 60% YoY, wrote Baird Equity Research analysts in a note published Monday. “Based on company disclosures and our analysis of Shopify’s global sales tracker, we estimate that total holiday weekend sales (Thursday through Sunday) totaled roughly $1.4 billion for Shopify, including $255 million on Thanksgiving, $573 million on Black Friday, $300 million on SMB Saturday, and $250 million on Sunday,” per the research note.